The present invention relates generally to electrical control systems and more specifically to a control for regulating the operation of a two-speed motor in a heat transfer system, such as an air conditioner or heat pump.
Typical air conditioners and heat pumps often include a thermostat and motor unit. The thermostat senses ambient temperature. When the temperature rises or falls beyond a predetermined standard, thus indicating that action is required by the motor unit, the thermostat will emit an electrical signal.
In many applications, the thermostat may emit at least two different signals, calling for two different types of action by the motor unit. If, for example, the ambient temperature is substantially different than a preset standard, the thermostat may emit a first, or high, signal calling for a high level of output by the motor unit. Conversely, if the ambient temperature is only slightly different than the desired temperature, the thermostat may emit a second, or low signal, calling for a lower level of output by the motor unit.
Many thermostats operate with "Class II" power. Class II power may be defined as only including electrical signals having a maximum R.M.S. voltage of 30 volts, and that are limited to a maximum current of 3.2 amperes.
The thermostat may be, for example, a multi-stage mercury bulb or a dual output electronic thermostat. Such thermostats may request no action by the motor unit, or high or low levels of action by the motor unit, or a change in the level of action by motor unit between high or low output states.
The motor unit may include both a compressor and an electric motor. The motor may run at either a high or low speed, depending on whether the motor receives a high or low speed input.
The compressor is operated by the motor and contains a predetermined amount of oil. When the motor and compressor begin operation, the compressor pumps a limited amount of the oil out of its crankcase. When the compressor continues running, the oil recirculates through the system, working back into the compressor crankcase. If, however, the compressor is started and stopped a number of times in quick succession, too much oil will have been pumped out of the crankcase for proper operation of the compressor. Thus, a rapid succession of attempts to start the compressor working may cause permanent damage to the compressor.
Similarly, changing the speed of the compressor, without allowing the compressor to first stop may damage the compressor. Also, starting problems may occur within the compressor, because time is required to let the refrigerant pressure equalize. Again, operation of the compressor under such a circumstance may cause permanent damage. Thus, the motor unit should be controlled to avoid operation under such conditions and thus avoid damage to the compressor.
Moreover, the operation of the electric motor unit driving the compressor should be controlled so that the motor will not operate if it is subject to a serious fault condition. For example, the rotor may become locked or the motor may be subjected to a severe overload or underload. Also, the line voltage applied to the motor may decrease substantially (during, for example, a "brownout") or the temperature of the motor windings may increase to an unacceptable level. Permanent damage to the motor may occur if it is allowed to continue operating under such conditions.